The present invention relates to a process for recovering transition metal catalysts. More particularly it relates to a process for recovering Group VIII noble metal catalysts in active form. Most particularly it relates to a process for recovering rhodium catalysts in active form.
Group VIII noble metal complexes are commonly used as homogeneous catalysts in a variety of organic reactions. Of these catalysts, the rhodium complexes are particularly useful in hydroformylation reactions in which an olefin is reacted with hydrogen and carbon monoxide in the presence of the catalyst to yield an aldehyde. Complexes of rhodium with organophosphorous ligands such as triphenylphosphine and triphenylphosphite are particularly attractive catalysts as they favour the formation of the desired aldehyde products. In some cases the selectivity for the desired aldehyde may be in the region of 90% or more when the appropriate phosphorous ligand is present.
However, whilst these catalysts are very effective, they suffer from a major drawback associated with their cost. It is therefore desirable to recover these highly expensive metals from the organic solutions in which they are removed from the reactor. Further, during operation, the catalyst may become deactivated and therefore needs to be removed from the reactor such that fresh active catalyst can be added. The removed catalyst will generally be reprocessed to recover the metal values.
The deactivated catalyst may have been thermally deactivated i.e. clustered and/or chemically deactivated i.e. poisoned or inhibited.
In some cases although the catalyst may be chemically active, the catalyst solution includes such a high concentration of non-volatile material that it is of no further practical use.
Although the mechanism of deactivation in aryl phosphine liganded systems by the formation of clusters is not entirely clear, it is believed that metal, e.g. rhodium clusters, having phosphido bridges may be formed, for example, by the loss of one or more phenyl groups from the aryl phosphine molecule. The chemical deactivation may be poisoning such as sulphur compounds, chloride, cyanide and the like.
The chemical deactivation may also be inhibition of the catalyst. Inhibitors that may be found in, for example, propylene and butylene hydroformylation include acetylenes and acroleins.
Conventionally, the operators of the plant have had to collect the active and/or inactive catalyst by shutting down the reactor, removing the catalyst in solution and concentrating it to partially separate it from the other components present. Additionally, or alternatively, catalyst may be collected from one or more reactor streams. By reactor stream we mean any stream which is obtained from any point in a process and which will contain Group VIII noble metal catalyst.
The Group VIII noble metals have conventionally been removed from the organic solutions by a variety of means before being shipped off-site for regeneration. This means that if the operation of the plant is not to be shut down for a prolonged period, the operator must purchase more of the very expensive catalyst to operate the plant than he actually requires at any one time.
There are also environmental issues associated with the regeneration of the catalyst where phosphorous ligands are present.
A variety of means of recovering the Group VIII noble metals from solution has been suggested including precipitation followed by extraction or filtration and extraction from the organic mixtures using, for example, amine solutions, acetic acid, or organophosphines. The organic solution of a deactivated solubilized catalyst may be treated to improve the extractability of the metal. Examples of this may be found in U.S. Pat. No. 4,929,767 and U.S. Pat. No. 5,237,106 which are incorporated herein by reference.
Ion-exchange methods have also been suggested, for example in U.S. Pat. No. 3,755,393 which describes passing a hydroformylation mixture through a basic ion-exchange resin to recover rhodium. A similar process is described in U.S. Pat. No. 4,388,279 in which Group VIII metals are recovered from organic solution using either a solid absorbent such as calcium sulfate, an anionic ion-exchange resin or molecular sieves.
An alternative arrangement is described in U.S. Pat. No. 5,208,194 in which a process is described for removing Group VIII metals from organic solutions which comprises contacting the organic solution with an acidic ion-exchange resin containing sulfonic acid groups. The treated solution is then separated from the ion-exchange resin and the metal values are recovered from the resin by any suitable means. The means that is suggested is that the resin should be burnt off in an ashing process which leaves the metal in a form suitable for recovery.
These prior art processes, whilst being suitable for separating the metal from the stream in which it was removed from the reaction, suffer from the disadvantage that the operator of the reactor must send the recovered metal concentrate off site to be converted into an active form. Further, where the stream removed from the reactor includes active catalyst, the separation procedure will either leave it in a form in which it cannot be returned to the reactor or will cause it to be deactivated such that it is no longer suitable for use in the reactor and removal off-site for regeneration is required.
In U.S. Pat. No. 5,773,665, a process is suggested which enables active catalyst contained in a stream removed from a hydroformylation process to be separated from the inactive catalyst and the active catalyst following treatment, to be returned to the hydroformylation reactor. In the process a portion of the recycle stream from the hydroformylation reaction is passed through an ion exchange resin column to remove impurities and active rhodium and the thus purified recycled stream, which may contain inactive catalyst, is returned to the hydroformylation reactor.
The impurities, which may include aryl phosphine oxide, alkyl phosphine oxide, mixed phosphine oxide and high molecular weight organic compounds, are removed from the resin by washing with, for example, an organic solvent. The effluent from this wash is removed as a waste stream. The active catalyst remains bound to the resin during this washing process.
The resin is then treated with a catalyst removal solvent such as isopropanol/HCl to produce a stream containing “active” rhodium catalyst for eventual recycling to the hydroformylation reactor. Whilst the catalyst has not been deactivated by thermal or chemical means and is therefore referred to as “active” it is not in a form in which it will actually act as a catalyst in the reactor. Thus, before the catalyst can be recycled it must first be removed from the resin using a strong acid reagent and then converted to the hydridocarbonyl by treatment with hydrogen and carbon monoxide in the presence of an acid scavenger and a ligand to make it a truly active catalyst.
In an optional arrangement, the inactive rhodium catalyst, i.e. the clustered catalyst, which passed through the ion-exchange resin without being absorbed and which is contained in the purified recycle stream may be reactivated by conventional technology such as by wiped film evaporation followed by oxidation and subsequent reduction before being returned to the reactor. Thus this inactive catalyst is not treated by the ion-exchange resin.
Whilst this process goes some way to addressing the problems associated with prior art processes, in that it suggests a means of separating the active catalyst on site, it suffers from various disadvantages and drawbacks in particular those disadvantages associated with the need to treat the “active” catalyst after it has been removed from the ion-exchange resin and before it can be returned to the reactor. Indeed it is the ion-exchange treatment which means that the catalyst is no longer suitable for use in the reactor.
Although in a preferred embodiment, U.S. Pat. No. 5,773,665 does suggest that the thermally deactivated catalyst may be regenerated before return to the reactor, the overall plant is expensive to construct and operate because of the number of separation and treatment steps, some of which occur in the presence of corrosive acid media, required to achieve full recycle. A further drawback associated with the presence of acid media is the further complexity and costs associated with the consumption of base required to neutralise the acid and dispose of the acid salts.
There is therefore a desire to produce a process for the recovery of Group VIII noble metal catalysts the plant for which is simple and cost-effective to construct and to operate.